There is a technique of performing communication by using an OFDM (Orthogonal Frequency Division Multiplexing) signal constituted of a plurality of subcarriers orthogonal to each other (for example, Patent Document 1).
An ordinary communication device (transmitter) for transmitting the OFDM signal is configured to perform a primary modulation for mapping transmission data on a complex plane to thereby obtain a complex symbol, and then perform an inverse fast Fourier transform (IFFT) on the complex symbol, to generate a baseband OFDM signal. Then, the communication device performs a predetermined process, such as a quadrature modulation and a frequency conversion, on the baseband OFDM signal, to generate a carrier-band OFDM signal. The communication device outputs the carrier-band OFDM signal as a communication signal to a channel.
On the other hand, an ordinary communication device (receiver) for receiving the OFDM signal is configured to perform a predetermined process, such as a frequency conversion and a quadrature detection, on the reception signal, to generate a baseband OFDM signal. Then, the communication device performs a demodulation process, such as a fast Fourier transform (FFT) and a demapping process, on the baseband OFDM signal, to modulate data.
In a case where communication is performed in packets, the receiver does not recognize when the transmitter transmits a packet signal, and in other words, when a packet signal arrives at the receiver. Therefore, the receiver has to stand by for the arrival of a packet signal, and detect a packet signal from signals received during the stand-by period. Such a process for detecting a packet signal is performed by using a preamble signal that is included in the packet signal.